Toshiba 49L621U

[budm]

Re: Toshiba 49L621U

The PWM pin on the power supply side has the pull-up resistor to pull PWM pin solid high when not connected to the main board, which is equal to 100% pulse width which is equal to full backlights level, the switched Transistor on the main board that is connected to the PWM Dim pin is switched on and off with PWM signal from micro controller.
Glad the main board fixed the problem.

[jostewcrew]

Re: Toshiba 49L621U

The PWM pin on the power supply side has the pull-up resistor to pull PWM pin solid high when not connected to the main board, which is equal to 100% pulse width which is equal to full backlights level, the switch Transistor on the main board that is connected to the PWM Dim pin is switched on and off with PWM signal from micro controller.
Glad the main board fixed the problem.

great explanation...thank you again

[jm173]

Re: Toshiba 49L621U

Hello,
I have same problem. My TV does not show any LED backlight. But i know its functioning since the chromecast function is working and I can hear the sound when i cast a video.
Also I checked the voltages.

As I turn on it goes like this

pin 1 vled1- = 35 v to 0.5 v
pin 2 nc
pin 3 vled2- = 35 v to 0.5 v
pin 4 nc
pin 5 vled3- = 35 v to 0.5 v
pin 6 nc
pin 7 nc
pin 8 nc
pin 9 nc
pin 10 nc
pin 11 vled+ = 108 v to 70 v
pin 12 vled+ = 108 v to 70 v
pin 13 vled+ = 108 v to 70 v

Can someone suggest what can i do? The cost of the power supply is a lot now since this is an old model. But I am searching if I can get a good deal.

[budm]

Re: Toshiba 49L621U

So what Voltage do you have on DIM pin of the power supply board, leave the black probe on the chassis, red probe on DIM pin then turn on the TV while watching the meter, also read post #59 and later.

[jm173]

Re: Toshiba 49L621U

Ya. This thread has been very helpful. Ya i did check that and it was 1.6v which was weird. Someone had mentioned somewhere to short BLON which is 3.5V and the DIM but no help.

[jm173]

Re: Toshiba 49L621U

Btw i did try disconnecting the PWM pin 15 and no change.

[jm173]

Re: Toshiba 49L621U

What is weird to me is the voltages I have (+70 v) is different from the baseline voltages that @jostewcrew had mentioned

[budm]

Re: Toshiba 49L621U

Do the same test on your post #83 but without LED connector in place, the Voltage should jump u higher than 108V on the VLED+ pins.

[jm173]

Re: Toshiba 49L621U

Nope. Still 69-70 Volts on all three + pins.
Tried again with turning off and on. Goes up to 130 then drops to 70v.

[budm]

Re: Toshiba 49L621U

Nope. Still 69-70 Volts on all three + pins.
Tried again with turning off and on. Goes up to 130 then drops to 70v.

That is what to expected, 70V is the feeding Voltage to the boost converter, the boost comes on but detects over Voltage so boost converter goes into shutdown.
BTW, you are 100 % sure that you do not see the backlights flash on at all when TV is first turned on?

[jm173]

Re: Toshiba 49L621U

I do see the backlight on momentarily at the start.
I wonder why is there over voltage. Maybe some leds are burnt or not working.

[nomoresonys]

Re: Toshiba 49L621U

That may be a match, as always triple check to try to ensure a match if buying:
https://www.tvpartstoday.com/PK101W1...k101w1230i.htm

[jm173]

Re: Toshiba 49L621U

Ya seems to be a match. I just want to make sure the issue is the power supply and not LEDs. It seems to be fine with the voltages. Any other suggestions before I buy the power supply.

[budm]

Re: Toshiba 49L621U

Btw i did try disconnecting the PWM pin 15 and no change.

So what is the Voltage on DIM pin of the power supply with just DIM pin wire not connected but other wires are still in place?
BTW, please provide straight shots pictures of your whole power supply board top and bottom side.

And since now we know that the backlghts do flash on for seconds, set meter to 200vdc scale, then keep black meter probe on the chassis, red probe on the pin 1 vled1-, then turn on the TV while keeping and eye on the meter and record what you see, turn the TV off and repeat the same test on vled2-, vled3-.

[jm173]

Re: Toshiba 49L621U

The pin 15 DIM value is 3.1 Volts. This is when it’s disconnected from connector.

As I turn my TV on the voltages go like this.
pin 1 vled1- = 35 v and steadily drops to 0.5 v. Stays at this
pin 2 nc
pin 3 vled2- = 35 v to 0.5 v
pin 4 nc
pin 5 vled3- = 35 v to 0.5 v
pin 6 nc
pin 7 nc
pin 8 nc
pin 9 nc
pin 10 nc
pin 11 vled+ = 108 v to 70 v
pin 12 vled+ = 108 v to 70 v
pin 13 vled+ = 108 v to 70 v

I’ll post pics tomorrow.

[budm]

Re: Toshiba 49L621U

It looks like the PWM controlled MOSFET on the Cathode return of the LED strings are not being turned on.
I need to see straight shots pictures of the whole top and bottom side of the power supply board to verify.

[nomoresonys]

Re: Toshiba 49L621U

Picture there if it's the correct one: https://www.shopjimmy.com/toshiba-pk...d-for-49l621u/

[jm173]

Re: Toshiba 49L621U

Sorry I got busy. But these are the correct images. Thanks @nomoresonys
I actually bought a used working power supply. But that one also is not working. Will try more things in the evening.

[jjanssen7]

Re: Toshiba 49L621U

I've repaired more than 15 TVs of this model. they have all come to me with backlight failure. LEDS come on for a flash, 5 seconds or even up to 5 minutes then BL shuts off but image remains on screen.

LED strips test OK with LED tester, but are failing under normal load. The way to find the one strip that is failing under load is to test each strip one at a time by plugging in a good known strip in the place of the one being tested. Once you find the failed strip, the BL stay on. It only a matter of time until the next strip fails... I suggest starting with a whole new set.

[nomoresonys]

Re: Toshiba 49L621U

Ok then, looks like they're prone to backlight failure, looks like it's time to do the dirty and strip it down.